This invention relates to totally enclosed, fan cooled induction motors. Such motors include stator and rotor totally enclosed to prevent damage from harmful substances in the operating environment and include a motor driven outer fan mounted on the rotor shaft projecting from the motor case at the non-drive end within a fan shield to direct cooling air over the outer surface of the case.
Within such motors, an internal fan is formed at each axial end of the rotor to encourage air circulation within the case with consequent heat transfer from the enclosed motor components to the inner surface of the case for heat transfer through the case to the cooling air outside. The most critical components to cool are the insulation of the stator windings and the bearings of the rotor shaft. The most difficult cooling is at the driven end of the motor, since it is farthest from the outer fan and the bearing at this end has a greater load.
Cooling problems of such motors in the past have been solved by such means as enlarging the case for more cooling surface area, rating the motors for a lower power rating for a given fan RPM and using a special and more expensive insulation with a higher temperature tolerance on the stator windings. However, the field has become extremely mature and competitive, so that there is a great benefit in seemingly slight improvements in cost or efficiency; and the aforementioned solutions sacrifice one or the other. It has become desirable to minimize the size and weight of the motor cases, use less expensive stator winding insulation and maximize the efficiency and power ratings of the motors.